The present invention relates to telecommunications in general, and more particularly, to a method for controlling reverse-link power control overshoot in a wireless communications system.
FIG. 1 depicts a schematic diagram of a portion of a conventional wireless communications system. As shown, the wireless communications system 1 includes a mobile switching center (MSC) 5, operatively connected with a plurality of base stations (BS) 10, for performing call connection control and mobility management. The base stations 10 include equipment to communicate with the MSC 5, and with various mobile stations (MS) 20. Each base station 10 transmits a pilot signal of constant power on the same frequency. The power level of the received pilot signal enables the mobile station 20 to estimate the path loss between the base station 10 and the mobile station 20. Knowing the path loss, the mobile station 20 adjusts its output transmit power such that the base station 10 will receive signals at the requisite power level. The base station 10 measures the output power of the mobile station 20 and informs the mobile station 20 to make any necessary adjustments to its transmit power. Mobile stations (MS) 20, operatively connected with the base stations 10, have equipment for allowing communication with the base stations 10, and for establishing forward links (base station to mobile station) and reverse links (mobile station to base station) therebetween.
CDMA is one scheme for defining forward and reverse link channels between base stations and mobile stations of a wireless communications system, such as a cellular telephone system. CDMA is an interference-limited system, whereby all mobile stations transmit at the same frequency, and the internal interference generated within the system plays a critical role in determining system capacity and voice quality. The CDMA system uses power control and voice activation to minimize mutual interference. Precise power control and voice activation avoids excessive transmitter power, which contributes to overall interference in the system. As the mobile station moves around, the wireless communications environment changes continuously due to fast and slow fading, shadowing, external interference, and other factors. One feature of a CDMA system is to transmit just enough power to meet the required performance objectives. The objective of power control is to limit the transmitted power on the forward and reverse links while maintaining link quality under all conditions. The transmit power from each mobile station must be controlled to limit interference therebetween. At the same time, the power level should be adequate for satisfactory voice quality. Otherwise, if more power is transmitted than necessary, one mobile station interferes with the operation of other mobile stations, and capacity is reduced.
In a CDMA system, all mobile stations 20 should achieve the same received power levels at the base station 10 to minimize interference within the wireless communications system 1 while achieving the required performance objectives. Reverse link power control affects the access and reverse traffic channels. Reverse link power control includes open-loop power control (also known as autonomous power control) and closed-loop power control.
Open-loop power control is based on the principle that a mobile station 20 closer to the base station 10 needs to transmit less power as compared to a mobile station 20 that is farther away from the base station 10 or is in fade. The mobile station 20 adjusts its transmit power based on total power received, including power received from all base stations 10 on the forward link channels. If the power received is high, the mobile station 20 reduces its transmit power, and vice versa. Dynamic or moving mobile stations (i.e., fading sources) in multipath require much faster power control than the open-loop power control can achieve.
Closed-loop power control provides additional power adjustments required to compensate for fading losses (e.g., when the signal interference between the mobile station and the base station increases). Closed-loop power control also provides correction to the open-loop power control. Once a traffic channel is established, the mobile station 20 and base station 10 engage in closed-loop power control. The quicker response time gives the closed-loop power control mechanism the ability to override the open-loop power control mechanism in practical applications.
FIG. 2 depicts a conventional method of reverse link closed-loop power control that is performed between the mobile station 20 and base station 10 to ensure that a proper communications link can be established therebetween. In step 100, the mobile station 20 transmits to the base station 10 at a certain output power in accordance with a request from the base station 10. In step 200, the base station 10 detects and measures the output power of the mobile station 20.
Next, in step 300, the base station 10 determines whether the output power of the mobile station 20 needs adjustment. This is done by comparing output power level requested by the base station 10 with the actual output power received from the mobile station 10, and considering the amount of signal interference in the current communications environment. If the mobile station""s output power does not require adjustment, the reverse-link power control process proceeds to step 600 to establish a communications link between the mobile station 20 and the base station 10 using well known techniques in the conventional art. In contrast, if the mobile station""s output power requires adjustment, the mobile station 20 calculates a new output power as shown in step 400.
Step 400 will be described in more detail with respect to the IS-95 CDMA standard. Under the current IS-95 CDMA standard, the transmit power of a mobile station 20 should be tightly controlled by the base station 10 through closed-loop power control. The base station 10 informs the mobile station 20, by sending a power control bit, to make any necessary adjustments (i.e., increase or decrease) to the mobile station transmit power. This is commonly referred to as the closed-loop power control correction. As known in the art, the mean output power of the mobile station 20 is calculated by the mobile station 20 using the following algorithm:                               Output          ⁢                      xe2x80x83                    ⁢          power                =                ⁢                                            -              mean                        ⁢                          xe2x80x83                        ⁢            input            ⁢                          xe2x80x83                        ⁢            power                    +          offset          +                      interference            ⁢                          xe2x80x83                        ⁢            correction            ⁢                          xe2x80x83                        ⁢            from            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            last            ⁢                          xe2x80x83                        ⁢            access            ⁢                          xe2x80x83                        ⁢            probe                    +                      (                                          NOM                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                PWR                            -                              16                *                NOM                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                PWR                ⁢                                  xe2x80x83                                ⁢                _                ⁢                                  xe2x80x83                                ⁢                EXT                                      )                    +                      INIT            ⁢                          xe2x80x83                        ⁢            _            ⁢                          xe2x80x83                        ⁢            PWR                    +                      the            ⁢                          xe2x80x83                        ⁢            sum            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            all            ⁢                          xe2x80x83                        ⁢            access            ⁢                          xe2x80x83                        ⁢            probe            ⁢                          xe2x80x83                        ⁢            corrections                    +                      the            ⁢                          xe2x80x83                        ⁢            sum            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            all            ⁢                          xe2x80x83                        ⁢            closed            ⁢                          xe2x80x83                        ⁢            loop            ⁢                          xe2x80x83                        ⁢            power            ⁢                          xe2x80x83                        ⁢            control            ⁢                          xe2x80x83                        ⁢                          corrections              .                                                          (        1        )            
In the above Equation (1), all terms are well known in the conventional art. To obtain the mobile station""s output power, a negative value of the mobile station""s mean input power is added with a predetermined offset value and the interference correction from the last access probe. Here, an access probe refers to a transmission during an access attempt performed between the mobile station 20 and the base station 10 to establish a communications link therebetween. Access probes are continuously performed at incremental power levels. This is referred to as performing access probe corrections. The next added term of equation (1) comprises a nominal power for handoff (NOM_PWR) multiplied with a nominal power for extended handoff (NOM_PWR_EXT). An initial power adjustment (INIT_PWR) is added thereafter. Then, the sum of all access probe corrections performed in establishing a communications links is added to the above-identified terms. Finally, the sum of all closed loop power control corrections is added.
Upon calculation of the mobile station""s output power, the mobile station 20 transmits to the base station 10 at the newly calculated output power, as shown in step 500. Steps 200 through 500 are repeated thereafter, as required, whereby closed-loop power corrections are continuously performed to establish and maintain proper communication between the mobile station 20 and the base station 10, as shown in step 600.
However, in the conventional art, there is a potential for power control xe2x80x9covershootxe2x80x9d (e.g., exceeding the maximum or minimum limit) of the mobile station""s transmit power in the reverse link in the conventional open-loop power control. The base station 10 may unnecessarily continue to inform the mobile station 20 to make adjustments to the mobile station""s transmit power, even when the mobile station transmit power limit has already been reached.
More specifically, due to open-loop correction and the maximum power constraint, the last term, i.e., the sum of all closed loop power control corrections, may be accumulated even when the mobile station transmit power has reached a maximum. Likewise, due to open-loop correction and the minimum power constraint, the last term, i.e., the sum of all closed-loop power control corrections, may be accumulated even when the mobile station transmit power has dropped down to a minimum. If the communications environment changes such that there is more or less signal interference between the mobile station 20 and the base station 10, the closed-loop power control will first need to cancel the previously accumulated power control correction values before it begins to have effect on the mobile station 20 to transmit at the appropriate transmit power.
For example, if the mobile station transmit power is already at its upper limit, and the communications environment changes such that there is increased signal interference between the mobile station and the base station, the base station 10 informs the mobile station 20 to further increase its transmission power. Thereafter, the mobile station 20 determines according to equation (1) a transmit power level greater than the mobile station""s maximum possible transmit power level; and therefore transmits at the maximum power level. If the communications environment changes again such that the mobile station 20 should transmit at less than the maximum power limit, the mobile station 20 will continue to transmit at the maximum power limit until the base station provides enough closed-loop power corrections to cancel out the previous closed-loop power corrections which increased the calculated transmit power level above the maximum power limit. Consequently, a power overshoot occurs, and the extra mobile station transmit power interferes with the transmission between other mobile stations 20 and the base station 10.
The same problem occurs with respect to the lower limit of the mobile station""s power level. If the mobile station transmit power is already at its lower limit, and the communications environment changes such that there is less signal interference between the mobile station and the base station, the base station 10 informs the mobile station 20 to further decrease its transmission power. Thereafter, the mobile station 20 determines according to equation (1) a transmit power level lower than the mobile station""s minimum possible transmit power level; and therefore transmits at the minimum power level. If the communications environment changes again such that the mobile station 20 should transmit at greater than the minimum power limit, the mobile station 20 will continue to transmit at the minimum power limit until the base station provides enough closed-loop power corrections to cancel out the previous closed-loop power corrections which decreased the calculated transmit power level below the minimum power limit. Consequently, the process of canceling out the previous closed-loop power corrections is time consuming, and thus degrades the reverse link performance.
Accordingly, power overshoot beyond the upper and lower transmission power limits at the mobile station 20 causes excessive interference and degrades overall reverse link performance.
In accordance with the present invention, a method for controlling reverse-link power control overshoot is achieved by recognizing that the mobile station has transmitter power limitations, and that unnecessary adjustments to the mobile station""s transmit power need not be made once the mobile station transmit power has already reached its limit. This is achieved by detecting whether the mobile station transmit power has reached its limit, and adjusting the mobile station mean output power by applying a constant value, to thus eliminate the need to cancel previously accumulated power overshoot corrections.